The field of the invention is impedance testing systems.
Transmitting a sinusoidal signal through a component results in an amplitude change and phase shift of the signal as a result of the impedance of the component. Although the impedance of a component can be measured, the measured impedance varies depending on the frequency of the signal being used. Moreover, it is impossible to accurately measure the impedance of a component without taking into consideration other factors such as the internal impedance of the measurement device (hereinafter xe2x80x9cmeterxe2x80x9d) being used to perform the measurement, the impedance of the conductive path/set of test leads used to transmit the signal through the component, and the affect the impedance of the component has on the signal being transmitted by the meter.
In order to compensate for the factors affecting impedance measurement, impedance meters are generally xe2x80x9ccalibratedxe2x80x9d using a fixed frequency signal, calibration loads having known impedances, and a particular set of test leads (xe2x80x9ctest lead setxe2x80x9d) which is to be used to couple the component to be measured to the meter. In such instances the test lead set is first used to couple a calibration load to the meter and various adjustments are made to the meter so that it provides an accurate reading of the known impedance of the calibration load despite the affects of the internal impedance of the meter and the impedance of the test lead set on the measurement. The current settings of the meter once it is calibrated are recorded (generally electronically within the meter) as a set for later use when measuring components having impedances comparable to the impedance of the load using the same frequency signal. As the actual impedance of a component can affect the signal being produced by the meter, and because impedances are a function of frequency, and because impedances are likely to vary between test lead sets, it is not uncommon to have several sets of calibration settings recorded and choosing a particular set for use based on the anticipated impedance value of the component, the frequency of the signal being used, and the test lead set being used.
Although such methods provide better measurements despite the affects of frequency, component impedance value, and impedances introduced by the meter and test lead set, they do not accurately compensate for any affects caused by subjecting the test lead set and meter to environmental changes. This inadequacy becomes particularly troublesome when measuring changes in the component""s impedance caused by environmental changes. Such measurements are generally part of life cycle testing of circuit boards during which typically involves subjecting the boards to repeated cycling between environmental extremes while concurrently measuring the impact such environmental changes have on the impedances of components imbedded within the circuit board. Although it is possible to isolate the meter from such changes, at least a portion of the conductive path formed by the test lead set will be subjected to the same environmental changes as the circuit board and its components.
One method for compensating for the environmental changes on the test lead set would be to recalibrate the meter/obtain a set of calibration settings in the desired environment. Doing so is problematic, however, because it introduces significant delays into the testing process by requiring that the test lead set be switched, typically manually, between the calibration load(s) and the component(s) to be tested. Moreover, such a switch may not be possible in severe environments. If such were the case, delays would be required to transition between an environment suitable for testing and the more severe test environment, and would possible affect the measurements by introducing a cycle between environments between calibration and component measurement steps.
Thus, there is a continuing need for improved methods and devices of testing, particularly in regard to compensation for impedance changes in caused by fluctuations of environmental factors.
Methods and apparatus for improved impedance measurements are provided which allow for relatively shorter recalibration delays during testing and which preferably eliminate the need to physically disconnect and reconnect test leads after initial calibration has been completed. In particular, an adjustment factor is calculated based on initial impedance measurements and is used to adjust future impedance measurements taken. Moreover, a plurality of loads having pre-measured impedances are switchably connected to the meter such that re-calibration using said loads preferably can be accomplished without manual connection or disconnection of test lead sets. The plurality of loads are preferably incorporated into a test board which also comprises additional test lead sets and a switching mechanism to alternately connect the various calibration loads and components under test to the meter. In a preferred embodiment, the board is constructed to minimize impedance differences between test lead sets.
Various objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the invention, along with the accompanying drawings in which like numerals represent like components.